234 BELL SYSTEM TECHNICAL JOURNAL 



tape or wire should be used is, in the end, an economic problem since 

 any disadvantage of one with regard to the other may be compensated 

 for by increasing the size of the copper conductor. 



Permalloy has another property which it is important to consider 

 in connection with its use on cables, namely, its great sensitiveness to 

 mechanical strain. Strain of deformation applied to it will modify 

 its magnetic characteristics, and very great changes in its permeability 

 for small magnetizing forces may be produced by strains well within 

 the mechanical elastic limit. Consequently in making the cable it is 

 necessary to insure that the permalloy shall be as free as possible from 

 strains of deformation. There are two principal ways in which the 

 permalloy used for loading may be subject to such strains. The 

 first comes in the manufacture of the loaded conductor and the second 

 in the laying of the cable. 



Since permalloy is so strain-sensitive it must be annealed after it 

 has been applied to the conductor. Accordingly the hard-worked 

 metal is wrapped around the copper conductor and the conductor is 

 thereafter passed continuously through a furnace, maintained at 

 approximately 900° C, and from the furnace into a cooling tube. 

 The lengths of the furnace and cooling tube and the rate of passage of 

 the conductor are so chosen as to insure that the loading material 

 will get the necessary softening in the furnace and will be cooled at 

 the proper rate in the cooling tube. Even though the permalloy is 

 thus annealed on the conductor, it still might well be subject to 

 considerable strain, since the copper, on being heated to such a high 

 temperature, expands more than the permalloy and tends to weld to 

 it and, on contracting, would bend the permalloy tape near the spots 

 where welding occurs. To prevent this action the loading material 

 is applied very loosely and means are taken to prevent adhesion of 

 the permalloy to the copper. In spite of the great sensitiveness of 

 the permalloy to mechanical strain, the loaded conductor after heat 

 treatment stands ordinary handling very well without much loss of 

 permeability. However, if it were insulated by the methods which 

 have been used in the past in making deep-sea cables, it would lose 

 much of its inductance on laying on account of the effect of the great 

 pressures to which a cable is subjected. 



To prevent reduction of the permeability and consequent loss of 

 inductance on laying, it is necessary to provide that pressure on the 

 insulating material shall produce only true hydrostatic pressure on 

 the permalloy with no tendency to deform it. This result has been 

 accomplished by vacuum-impregnating the permalloy-loaded con- 

 ductor with a semi-fluid compound which fills all the interstices of 



